The present invention relates to image display apparatuses which provide an image displayed on an image display element such as an liquid crystal display element (liquid crystal panel) to an observer through an optical system. The image display apparatuses are especially suitable for head mounted displays (HMDs).
The HMD needs to increase realistic sensation by widening the view angle while reducing its size. In addition, considering the weight balance of the HMD, reduction of the thickness of the HMD is desired.
Increasing the size of the liquid crystal panel is preferable for responding the requirement for a wider view angle. However, enlarging the panel increases the cost of the HMD. To achieve a wider view angle by using a small panel, intermediate image-forming optical system which forms an intermediate real image of an original image displayed on the panel and provides an enlarged virtual image of the intermediate image to an observer is preferable to an optical system without forming an intermediate real image. However, the once-image-forming optical system includes a lot of components and its optical path length is long, so that the HMD generally is heavy and large.
An HMD designed to have a wide view angle and a small size has been disclosed in Japanese Patent Laid-Open No. 2003-149593 (corresponding to EP1312968A1). In this HMD, a shuttle optical path is formed by using a returning reflecting surface and decentered reflecting surfaces located anterior and posterior to the returning reflecting surface to form a long optical path which is needed for once image formation in a small optical system.
In addition, the HMD needs to provide a good appearance of an observer wearing it on his/her head. Especially when the observer wearing an HMD is seen from the front, an HMD having a thin and horizontally long shape provides a better impression if the HMD has the same projected area as that of other ones.
For example, when an observer shown in FIG. 7A wears an HMD on his head, the observer wearing an HMD having a vertically short and horizontally long shape as shown in FIG. 7C looks better than the observer wearing an HMD having a vertically long and horizontally short shape as shown in FIG. 7B. To construct such a horizontally long HMD, using a horizontally-folded optical system in which plural decentered reflective surfaces (the decentering directions thereof) are arranged in the horizontal direction of the face has an advantage over using an optical system in which the reflecting surfaces are arranged in the vertical direction of the face.
Japanese Patent Laid-Open No. 2004-341324 has disclosed a horizontally-folded optical system which uses the shuttle optical path formed with the returning reflecting surface disclosed in Japanese Patent Laid-Open No. 2003-149593. Japanese Patent Laid-Open No. 2004-341324 has disclosed in its embodiment an optical system in which a reflecting surface is located between an optical element that forms the shuttle optical path and an image display element and in which optical paths for right and left eyes intersect with each other between the image display element and the reflecting surface and between the reflecting surface and the optical element.
The optical system disclosed in Japanese Patent Laid-Open No. 2004-341324 makes it possible to construct a horizontally long HMD. However, its thickness to the front of the observer is not sufficiently reduced.